1. Field of the Invention
The present invention relates to an image reading apparatus which reads an image formed on an original while conveying the original and a shading correction data generating method applied to the image reading apparatus.
2. Description of the Related Art
Conventionally, when reading an image of an original in an image reading apparatus or the like, the shading correction has been conducted to correct the variation in sensitivity of light receiving elements of the line image sensor or to correct the unevenness of the applied light intensity resulted from the directivity of light source or the like.
During the shading correction, when the shading correction is executed using shading correction data acquired by a line image sensor reading a white reference plate for shading correction arranged at farther/closer position than the original reading position (in-focus position), the shading correction data needs to be converted into ideal shading correction data which must be acquired by reading the white reference plate for shading correction arranged at the same position as the original reading position. This conversion is carried out by multiplying the shading correction data by light intensity distribution correction coefficients acquired in advance.
One example of the method of generating the light intensity distribution correction coefficients includes the following. A reference sheet made of materials having equivalent whiteness to the white reference plate for shading correction is arranged in advance at the position approximately equal to the original reading position (in-focus position). The white reference plate for shading correction and the reference sheet are read using the line image sensor. The read data of the white reference plate and the read data of the reference sheet are sampled, and the light intensity distribution correction coefficients expressed by the differences calculated from comparison of two read data are obtained (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2006-072838).
However, the read data of the white reference plate for shading correction and the read data of the reference sheet read by the line image sensor contain noise resulted from the variation in sensitivity of light receiving elements, or noise caused by stain or the fracture of the reference plate or the reference sheet that causes fluctuation of the received light intensity of light receiving element. Therefore, the light intensity distribution correction coefficients calculated from the differences obtained from comparison of the read data of the white reference plate for shading correction and the read data of the reference sheet contain noise having large margin of errors. Furthermore, depending on the construction of the light source, the light intensity distribution correction coefficients vary depending on the positions of the light receiving elements of the line image sensor, due to the directivity of the light beam emitted by the light source. For example, in the case of arranging LED at the end of the line image sensor in the longitudinal direction and guiding the light emitted by the LED to the entire reading positions of the line image sensor using a light guide member, the light intensity distribution correction coefficients significantly vary depending on the positions of the light receiving elements of the line image sensor.
FIG. 9 is a view showing one example of light intensity distribution correction coefficients when a white reference plate for shading correction is arranged at a position farther than the original reading position (in-focus position) opposing the line image sensor. And, also the light intensity distribution correction coefficients are obtained from ratio of two read data described above. As shown in FIG. 9, part of the coefficients are particularly high in the pixels corresponding to the light receiving elements near the end of the line image sensor (α section in FIG. 9), because there is a difference between the position of the white reference plate for shading correction and the position of the reference sheet. This characteristic called unevenness caused by directivity is particularly evident when the LED is arranged at the end of the line image sensor in the longitudinal direction to guide, using the light guide member, the light emitted by the LED to the entire reading positions of the line image sensor. The unevenness caused by directivity is not significantly evident in the read data at the point of acquiring the read data of the white reference plate for shading correction and the read data of the reference sheet. However, the unevenness caused by directivity becomes evident in the data of the light intensity distribution correction coefficients calculated using the ratios of the read data of the white reference plate for shading correction to the read data of the reference sheet. Meanwhile, when strong filtering is performed to remove the undesired noise that is contained in the light intensity distribution correction coefficients and that causes large errors, the peak indicative of the uneven directivity required in the shading correction is lost from the data of the light intensity distribution correction coefficients, which makes it impossible to perform the accurate shading.
Even with an assumption that the line image sensor is an ideal one which does not cause any variation in sensitivity of light receiving element, the light intensity distribution correction coefficients contain noise caused by stain on a contact glass, stain on the reference plate, or the like (for example, β section in FIG. 9). In order to remove such noise, low-pass filtering with a narrow passband needs to be performed using a noise filter having a relatively large mask width (mask size). However, as in the case mentioned above, the unevenness caused by directivity required in the shading correction is also removed when a noise filter having a large mask width is used.